In the following description, it is first defined that the term “forward” refers to a signal transfer direction from a base station to a terminal and the term “reverse” refers to a signal transfer direction from a terminal to a base station.
A TDD radio repeater apparatus serves to extend a radio transmission/reception section of a base station, and is equipment to be capable of very effectively providing mobile communication services at inexpensive cost in radio wave shadowing areas such as the inside of building, underground and the like. Therefore, this TDD radio repeater apparatus has been frequently used for increase of subscriber capacity and for services of specific areas.
Further, the TDD radio repeater apparatus is a device which periodically performs ON/OFF operations of a signal transfer from a base station to a terminal (forward) and of a signal transfer from a terminal a to a base station (reverse) at regular time intervals using a same transmission/reception frequency. That is, the TDD radio repeater apparatus carries out an operation in which a down link that receives a radio wave from a base station and radiates it to a service area and an up link that receives a radio wave from a service area (terminal) and transmits it to a base station are alternatively ON/OFF. The ON/OFF operations are made by the control of a synchronization acquisition unit so that a signal is transferred from a base station to a terminal or from a terminal to a base station by a switching unit which is arranged at each of donor and service antenna ends.
At this time, since a TDD method is used for the purpose, there is a need for the process of synchronization acquisition for the down link from the base station to the terminal in order to synchronize the ON/OFF operation timings of the up link and the down link. This synchronization acquisition process is normally made by using a preamble of the down link. Since the preamble is structured to effectively establish the synchronization acquisition in the TDD method and has a base station′ identification (ID) and segment information, it can be obtained through the signal processing process of the synchronization acquisition unit.
FIG. 1 illustrates a block diagram of a conventional TDD radio repeater apparatus.
As shown in FIG. 1, the conventional TDD radio repeater apparatus includes a donor antenna 100 for transmitting/receiving a signal connected to a base station; a donor band pass filter 101 for band-filtering the signal transmitted/received through the donor antenna 100; a donor switching unit 102, placed between the donor band pass filter 101 and a first down converter 103 and a second up converter 112, for switching the transmitted/received signal under the control of a synchronization acquisition unit 105; the first down converter 103 for down-converting the received signal from the donor switching unit 102; a first band filter 104 for filtering the received signal down-converted by the first down converter 103; the synchronization acquisition unit 105 for accepting the received signal from the first band filter 104 to acquire a synchronization of a TDD radio repeater signal; a first up converter 106 for taking the received signal from the first band filter 104 and up-converting the down-converted repeater signal (received signal) into an original repeater signal; a service switching unit 107, disposed between each of the first up converter 106 and a second down converter 110 and a service band pass filter 108, for switching the transmitted/received signal under the control of the synchronization acquisition unit 105; the service band pass filter 108 for band-filtering a signal transmitted/received through a service antenna 109; and the service antenna 109 for transmitting/receiving a signal connected to a terminal of a service area. Here, a reverse operation (transmission of a signal from the terminal to the base station) is similar to the above-mentioned forward operation and thus will easily be understood by a person skilled in the art with reference to the above description and FIG. 1. Therefore, details thereof are omitted here.
In the conventional TDD radio repeater apparatus, in case where the power of signal outputted from the donor antenna 100 is strong, an output signal is feedbacked to the service antenna 109 and again amplified and radiated through the donor antenna 100 and oscillated. This case is also applied to the inverse process. To minimize this oscillation phenomenon, the donor antenna 100 and the service antenna 109 are installed at far distance so that the degree of isolation between the two antennas is larger than a given gain. Like this, in case where the two antennas are arranged at far distance, there exists a problem that the installation cost is increased. Further, in order to fully reduce the feedback phenomenon (oscillation problem) of a transmitted signal, it is followed by the design constraint that the gain of the radio repeater apparatus should be lowered.
To solve the above problem, there has been proposed a radio repeater apparatus as shown in FIG. 2.
FIG. 2 illustrates another example of a conventional radio repeater apparatus, especially a structure of a down link.
In the radio repeater apparatus shown in FIG. 2, since the synchronization acquisition procedure is the same as that described in FIG. 1 and the up link and the down link are identical in structure to each other, only the down link will be described, without the switching unit.
A difference between the radio repeater apparatus shown in FIG. 2 and the TDD radio repeater apparatus in FIG. 1 is that the former apparatus has the function of preventing the oscillation problem, without decreasing the amplification gain of the radio repeater apparatus, by estimating a phase and magnitude of a signal feedbacked to a receiving antenna by using a specific tone and removing the feedback signal from a received signal through the receiving antenna based on the estimated phase and magnitude of the signal.
The radio repeater apparatus of FIG. 2 performs an initial operation function which estimates a phase and magnitude for a specific tone and decides a reverse phase to remove a feedback signal based on the estimated phase and magnitude, and a normal operation function which removes and radiates the feedback signal from an actual repeater signal according to the reverse phase decided in the initial operation state.
First, a microprocessor 230 waits for a synchronization signal for a down link from a synchronization acquisition unit 231 in the initial operation state. In the TDD radio repeater apparatus, it is required that the synchronization acquisition with a base station be preceded, and when the synchronization acquisition is made, the microprocessor 230 generates a specific tone based on a synchronization signal through a transmitting end tone generator 220. Then, the generated specific tone is filtered by a transmitting end band pass filter 222 and then radiated to a service area through a transmitting end antenna.
Next, when a transmitted signal is induced (feedbacked) to and received by the receiving end antenna, it is filtered by a receiving end band pass filter 200, amplified by a receiving end amplifier 201, and then inputted to a phase synthesizer 211 which is an original signal path. At this time, a variation of each of the phase and magnitude of the specific tone is detected by a receiving end tone phase/magnitude detector 202 which is arranged at the front end of the phase synthesizer 211. The values so detected are then to a phase/magnitude comparator 210. Then, the phase/magnitude comparator 210 compares the output values of the receiving end tone phase/magnitude detector 202 with a phase and magnitude of a specific tone obtained from a transmitting end tone phase/magnitude detector 223 which is disposed at a next stage of the transmitting end band pass filter 222, and provides the comparison result to the microprocessor 230.
Then, the microprocessor 230 transmits a reverse phase value for the specific tone, which is obtained by using the magnitude and phase variations for the specific tone, to the transmitting end tone generator 220 and the reverse phase synthesizer 212. According to this, the reverse phase synthesizer 212 generates a specific tone with the reverse phase by using the reverse phase value from the microprocessor 230 and the specific tone internally feedbacked through a gain attenuation/feedback path part 213 and a distributor 214, and transfers it to the phase synthesizer 211 to remove the specific tone from the received signal.
By the above operation, when the specific tone (feedback signal) is removed from the received signal, a receiving end tone detector 215 measures a magnitude of the specific tone, and if the measured magnitude becomes less than a preset magnitude, notifies the microprocessor 230 of this and the initial state operation is finished.
Thereafter, in the normal operation state, the radio repeater apparatus receives and amplifies a radio wave from the base station and then radiates it to the service area. Then, it interrupts the operations of the transmitting end tone generator 220, the receiving end tone phase/magnitude detector 202 and the phase/magnitude comparator 210, and operates the reverse phase synthesizer 212 and the phase synthesizer 211, to thereby remove the feedback signal by using the phase and magnitude for the specific tone which is obtained in the initial operation state. At this time, the received signal that passes through the phase synthesizer 211, which the feedback signal is removed, is amplified by the transmitting end amplifier 221, filtered by the transmitting end band pass filter 222, and radiated through the transmitting antenna.
Further, the receiving end tone detector 215 watches an output of the received signal that continuously passes through the phase synthesizer 211, which the feedback signal is removed, judges it to be oscillation if the output of the received signal is detected to be greater than a given level, and provides the judgment result to the microprocessor 230. Then, the microprocessor 230 blocks the RF switching unit 216 to block the connection passageway of the phase synthesizer 211 and the transmitting end amplifier 221, thereby making the initial state operation performed again.
The radio repeater apparatus as shown in FIG. 2 removes the feedback signal by finding out the phase and magnitude thereof if the signal radiated to the service area by using the specific tone is feedbacked back to the receiving antenna, to thereby prevent the oscillation of the signal. However, this radio repeater apparatus of FIG. 2 has a drawback in that it cannot repeater an actual signal during the process of detecting the phase and magnitude by using the specific tone.
Furthermore, under the radio environments, the phase and magnitude of the feedback signal are continuously varied due to change of peripheral environments, such as a case where peripheral objects are moving or buildings are under construction, and thus the optimal phase and magnitude are also varied. Therefore, it is required that the optimal phase and magnitude be continuously updated to remove the feedback signal. If it does not do so, there exists a feedback signal component even after removing the feedback signal. As a result, the radio repeater apparatus of FIG. 2 has a problem that a signal is oscillated by the residual feedback signal component.
In particular, the radio repeater apparatus of FIG. 2 carries out its operation for each of the initial operations state and the normal operation state separately. Therefore, if it fails to accurately predict the phase and magnitude in the initial operation state, the performance of the apparatus is lowered in the normal operation state, and it does not cope with abrupt external environment change. Moreover, since it is not possible to adjust the output magnitude, stability cannot be guaranteed, even in the change of the sudden radio channel environment, and the quality of output between the base station and the mobile communication terminal cannot be maintained at required levels.